The present invention relates to automatically controlled cutting machines for cutting sheet material with sharpenable blades. More particularly, the invention is concerned with cutting machines that are program controlled and guide a cutting blade which is sharpened periodically during a cutting operation.
The introduction of numerical control to the field of cutting sheet materials such as woven and non-woven fabrics, cardboard, leather, paper and other limp sheet material has improved production, reduced material waste and insured accurate high quality cutting. In a typical numerically controlled cutting machine such as shown in U.S. Pat. No. 3,495,492 having the same assignee as the present invention, limp sheet material is positioned in a multi-ply layup on a penetrable bed of foamed plastic or bristled mats, and a reciprocating cutting blade is plunged through the layup and translated relative to the bed under program control. The program contains information in numerical form defining cutting paths to be followed by the blade as the cutting operation is carried out.
Since a large volume of cutting can be carried out by a programmed cutting machine, it is not uncommon to cut pattern pieces for garments, upholstery and other products from multi-ply layups which are, for example, 10 feet (3 meters) in width and 100 feet (30 meters) or more in length. Normally cutting blades made of even the hardest tungsten carbide or steel require sharpening many times as a layup of such size is cut. It has become a common practice in certain machines to employ automatic blade sharpeners carried with the cutting blade to sharpen the blade periodically. For example, in the programmed machine, linear displacement of the blade in cutting engagement with the sheet material can be monitored and accumulated and after 100 inches (2.5 meters) of lineal cutting is detected, the sharpener is energized to sharpen the cutting edge of the blade. Cutting is then resumed and the sharpening cycle is repeated when another 100 lineal inches of cutting have been completed.
It will be understood that in a large layup having a multitude of closely packed pattern pieces, the total lineal footage of cutting may require the cutting blade to be sharpened so many times that wear on the cutting blade results in a noticeable recession or shifting of the cutting edge relative to the tool axis. It is the tool axis which is referenced to and positioned by the cutting program. Accordingly, displacements of the cutting blade relative to the sheet material do not extend to points on the cutting path defined in the cutting program, and instead, the displacements terminate in the sheet material at positions falling short of the programmed positions by an amount equal to blade wear. Even if such wear only amounts to 0.06 inches (1.5 mm), the shortfall in reaching a programmed point can be significant where the average machine error is one half of this value. The result is a loss of accuracy or in situations where the line segments defining the cutting path extend in substantially different directions, the pattern pieces may not be completely severed from the layup. Thus, the pattern pieces may be torn during removal from the layup or corners and notches may be incorrectly formed.
It is, accordingly, a general object of the present invention to provide method and apparatus for cutting limp sheet material in accordance with a program without the defects that arise due to wear of the cutting edge on a blade.